Zinc smelting



June 29, 1954 s. E. wooDs 2,682,462

ZINC SMELTING Filed Aug. 14. 1953 2 Sheets-#Sheet 2 Patented June 29, 1954 ZINC SMELTING Stephen Esslemont Woods, Bristol, England,'assignor to The National Smelting Company Limited, London, England Application August 14, 1953, Serial No. 374,322

Claims priority, application Great Britain June 17, 1949 6 Claims.

` This invention relates to zinc smelting in which most of the zinc is recovered in the form of metal, and has for its object the provision of certain improvements for minimizing the reoxidation of zinc vapor in the gaseous product of the smelt,- ing operation during the transfer thereof to a zinc vapor condenser. More particularly, the invention is concerned with the smelting of Zinc ores in a blast furnace and aims to effect the transfer of the blast furnace gases from the smelting zone of the furnace to a zinc vapor condenser with substantially negligible reoxidation of rinc vapor.

The gaseous product of the zinc smelting operation contains zinc vapor (Zn), carbon dioxide (CO2) and carbon monoxide (CO) in varying relative proportions, depending upon the character of the smelting operation. For example, the gaseous product of a blast furnace -smeltng operation (herein called the blast furnace gases) may contain (approximately) 5% Zn, 6.5% CO2, 26.5% CO and 62% nitrogen (N), while the gaseous product of an electrothermic smelting operation may contain (approximately) ,419% Zn, 1.5% CO2, 51% CO and 7.5% N. The relative proportions of Zn, CO2 and .CO in the gaseous product of each of these operations is such that zinc vapor reacts with carbon dioxide to form zinc oxide `(ZnO) when the temperature .of .the gaseous product falls to about 941 C. The oxidation of zinc Vapor by carbon dioxide takes place in accordance with the following reversible reaction:

In a gaseous product containing the three gases (Zn, CO2 and CO) the temperature at which zinc Vapor is oxidized by carbon dioxide (herein called the reoxidation temperature) depends on the product of the zinc vapor and carbon dioxide concentrations divided by the concentration of carbon monoxide, expressed by the following equilibrium ratio:

The higher the equilibrium ratio, the higher is the reoxidation temperature.

While vthe reoxidation temperature (941 C.) of the aforementioned gaseous products `of zinc smelting operations is usually lower than the normal initial temperature of the :gaseous product, the temperature of the gaseous product `falls in the course of its conventional travel from the smelting furnace to the condenser below the reoxidation temperature `with attendant troublesome formation of zinc oxide. In other Words, the difference between the reoxidation temperature and the normal initial temperature of the gaseous product is usually insuflicient to compensate for the loss of heat during the transfer of the gaseous product to the stage of zinc vapor condensation and the temperature of the gaseous product falls below the reoxidation temperature. The present invention aims to overcome this difficulty by heating the gaseous product to increase this temperature diiferential. Thus, in accordance with the invention, the gaseous product is heated to a temperature substantially above its normal initial temperature by directly introducing a controlled amount of an oxygencontaining gas, such as air, oxygen or oxygenenriched air, into the product after its discharge from the smelting zone of the furnace. By the resulting oxidation of carbon monoxide in the gaseous product, the temperature of the product is sufficiently raised to permit its transfer to the condenser at a temperature above that at which any appreciable amount of zinc vapor reacts With carbon dioxide.

The invention is of special advantage and value in the blast vfurnace smelting of zinc ores where the blast furnace gases contain a relatively large amount of nitrogen, a relatively smaller amount of carbon monoxide, a relatively small amount of zinc vapor, and carbon dioxide in amount comparable with and generally somewhat greater than that of the Zinc Vapor. The condensation of the zinc vapor content of the gases is preferably eflected in a condenser of the shock-chilling type, such as described in the U. S. patent of Robson et al. No. 2,464,262, patented March l5, 1949, and in the copending U. S. patent application of Robson and Derham Serial No. 80,91 6, lfiled March 1l, 19419, ln accordance with therpresent invention, the formation of zinc oxide between the blast furnace and the condenser, resulting from the fall in temperature of the blast furnace gases, is prevented or reduced by introducing a controlled amount ,of an oxygen-containing gas whereby to oxidize carbon monoxide, the heat of oxidation thus evolved serving to raise the temperature of the gases. By so introducing a controlled amount of the oxygen-containing gas into the blast furance gases at any convenient stage between the condenser and the place of their discharge from `the smelting Zone of the furnace, preferably as near as practicable to the latter place, the temperature ofthe gases is so raised that in .their subsequent travel to the stage of izinc vapor condensation by shock-chilling the loss in temperature of the gases is insufficient to lower their temperature to the reoxidation temperature and hence no appreciable amount of zinc vapor is oxidized by carbon dioxide.

While the inclusion in the blast furnace gases of slightly more carbon dioxide and slightly less carbon monoxide as the result of the oxidation of vcarbon monoxide by the introduced oxygencontaining gas slightly raises the reoxidation temperature at which Zinc vapor reacts with carbon dioxide, the additional heat imparted to the gases is sufficient to permit their travel and delivery to the shock-chilling zone of the condenser at a temperature above that at which any appreciable amount of Zinc vapor reacts with carbon dioxide in the gases. For example, in the blast furnace smelting of zinc ores, the initial gases commonly have a temperature of around 950 C. and contain around 6-8% of carbon dioxide and generally a somewhat smaller amount of zinc vapor (e. g. -6%). While this normal initial temperature of the gases is slightly higher than that at which zinc vapor will react with carbon dioxide therein, the temperature of the gases during their travel and delivery to the condenser will fall to a point at which some zinc vapor is oxidized by carbon dioxide. By introducing an oxygen-containing gas into the initial blast furnace gases, in accordance with the invention, the temperature of the gases can readily be raised to a temperature above 1000? C., which is sufficiently high to compensate for the heat losses incident to the travel of the gases to the condenser without lowering their temperature to the critical reoxidation temperature. In practicing the invention, the carbon dioxide content of the blast furnace gases can be increased to and even somewhat higher, with no deleterious effect on the subsequent condensation of zinc vapor.

The oxygen-containing gas, particularly when containing a large 4volume of an inert diluent like nitrogen in air, should be preheated before introduction into the blast furnace gases. Preferably, the oxygen-containing gas is introduced into the blast furnace gases as close as practical to their discharge from the smelting zone in the furnace. A plurality of inlets for the oxygencontaining gas may be provided to ensure even distribution.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which Fig. 1 is a vertical section of a Zinc blast furnace of the type described in. the copending United States patent application of Derham Ser. No. 158,190, filed April 26, 1950, and

Fig. 2 is a vertical section of a zinc blast furnace in which the blast furnace gases are withdrawn from the top of the furnace above the charge level therein.

Referring to Fig. 1 of the drawings, the Zinc blast furnace is bounded by side walls I and 2. The zinciferous gases generated are withdrawn by an inverted trough 3. As the gases enter the passage or flue 4, they are mixed with a small proportion of air, oxygen or oxygen-enriched air, preferably preheated, introduced through the pipe 5 or a number of such pipes.

It will be understood that the resulting increase of carbon dioxide due to the combustion of the carbon monoxide in oxygen will tend to increase the formation of zinc oxide but it can be arranged that this effect is very much outweighed by the resulting increase in temperature of the gases.

If the gases were subsequently cooled down slowly then of course the increased carbon dioxide content would increase the tendency for reoxidation of the zinc; but it is already known to shock-chill the gases in the condenser in order to overcome this difficulty (see for Example U. S. Patent No. 2,464,262) so that the present invention is only concerned with preventing reoxidation during transit from the blast furnace to the condenser.

The invention will be further explained in the following examples:

Example 1 Initial composition of furnace gases:

Zinc per cent 5 Carbon dioxide do 6.5 Carbon monoxide do 26.4 Nitrogen do 62.1

Initial gas temperature=941 C.

Assume 1% by volume of oxygen at 25 C. is introduced into the blast furnace gases and is converted to carbon dioxide by combination with carbon monoxide in the gases.

It is found that the heat generated will raise the gases to a temperature of 1,098 C.

Owing to the increase in proportion of carbon dioxide the temperature at which the formation of zinc oxide can occur will be slightly higher than it was before the admission of the oxygen. Thus if the 941 C. is the temperature at which zinc oxide formation occurred initially then after the admission of the oxygen owing to the changes in the partial pressures a zinc oxide formation will occur at 965 C. Thus on the balance a considerable improvement has been effected as 'regards the prevention of zinc oxide formation.

Example 2 Initial composition of furnace gases as in Example 1.

3% of aircontaining 21% oxygen and 79% nitrogen is introduced.

If the reoxidation temperature and initial gas temperature are initially 941 C. then the reoxidation temperature becomes 955 C. while the gas temperature becomes 1,019 C.

The improvement is somewhat less than with the first example but if preheating is used more favorable results are obtained and ordinary air becomes almost as effective as oxygen, or oxygenenriched air.

Obviously, the pipe 5 may be positioned to introduce the oxygen-containing gas in the space beneath the inverted trough 3, and above the smelting charge below the trough.

In the zinc blast furnace 6 of Fig. 2, the blast furnace gases accumulate in and are withdrawn from the top of the furnace above the charge level a. The smelting charge, preheated to a temperature of at least 700 C. and preferably to about 850 C., is introduced into the top of the blast furnace, through a charging device 1 which may conveniently be of the double bell (or double slide or slide and bell) type to seal the top of the furnace. The blast furnace gases are withdrawn through a gas offtake 8 into a passage or flue 9 and thence suitably conveyed to the zinc condenser.

In accordance with the present invention, a controlled amount of an oxygen-containing gas, such as air, oxygen-enriched air, or oxygen, is n- 5. troduced through a pipe I into the blast furnace gases accumulating 'in thespace above the level of the charge, in vorder to raise the temperature of the gases, say to at least 1000 C.,by the oxidation of carbon monoxide. After such heating, the blast furnace gases will contain about 1% Amore carbon dioxide and about 2% less carbon monoxide, while the zinc content will remain approximately the same. With a blast furnacegas having an initial normal temperature of 941 C. and the following composition Per cen-t Zinc Carbon dioxide 6.5 Carbon monoxide 26.4 Nitrogen 62.1

3% of air (based on the volume of the blast furnace gases at normal temperature and pressure) heated to about 600 C. when introduced through the pipe l0 above the charge in the furnace will raise the temperature of the gases to about 1030" C. Using gases of higher oxygen content than air, the same increase in temperature of the blast furnace gases can be attained with a correspondingly smaller volume of the oxygen-containing gas. For example, about 0.5% of oxygen will effect about the same increase in temperature of the blast furnace gases as the aforementioned 3% of preheated air. Preheating of oxygen is unnecessary, since it contains no diluent and its cooling effect is negligible. The oxidation of carbon monoxide by the oxygen content of the oxygen-containing gas sufficiently raises the temperature of the blast furnace gases (e. g. to at least 1000 C.) to permit their travel and delivery to the shock-chilling zone of the condenser at a temperature above that at which any appreciable amount of zinc vapor reacts with carbon dioxide in the gases.

The blast furnace gases accumulate in the top of the furnace above the charge level at approximately atmospheric pressure. A controlled small proportion (say 5 up to 25%) of the oxygen-containing gas may be advantageously introduced into the blast furnace gases through a pipe Il entering the charging device 1. While the charging device is intended to provide a hermetical seal at the top of the blast furnace, in actual operation the seal cannot be maintained perfect, and there is a tendency for blast furnace gases to seep through the seal into the charge-holding space between the two bells, with attendant objectionable formation of rock zinc oxide upon the inner walls of the charging device. By introducing a small proportion of the preheated oxygencontaining gas into the space between the bells at a pressure slightly above that of the blast furnace gases such seepage is effectively prevented. The high temperature of the blast furnace gases resulting from the introduction therein of the controlled amount of oxygen-containing gas also effectively minimizes the formation of accretions of rock zinc oxide approximate the gas offtake and the delivery end of the charging device, which accretions would otherwise form and would have to be periodically knocked or scraped off.

Part or even all of the oxygen-containing gas may be introduced into the blast furnace gases a short distance below the charge level a, but above the active smelting vzone of the furnace. The gases rising through the upper part of the charge in the furnace have attained approximately their ultimate composition, and the introduction of the oxygen-containing gas into such upper part of the y6 charge serves to raise the temperature of the gases in accordance with the principles of the invention.

The blast furnace gases, heated as hereinbefore described to a temperature substantially above their normal initial temperature, are transferred through the flue system 9 to a condenser of the shock-chilling type, such for example as the lead splash condenser described in the copending United States patent application of Robson and Derham, Serial No. 80,916, filed March 11, 1949. The ue system is of the heat-insulated type to minimizeany substantial loss of heat in the gases in the course of their travel from the blast furnace yto the shock-chilling zone of the condenser, so that, even though the carbon dioxide content of the gases may have been raised to around 10% by the aforementioned oxidation of carbon lmonoxide, there is practically no tendency for zinc vapor to react with carbon dioxide in the gases.

While I have hereinbefore particularly described the invention with reference to the yblast furnace smelting of zinc ores, the invention as well as the underlying principles of the foregoing discussions are equally applicable to other types of zinc smelting more particularly where the gaseous product of the smelting operation contains at least 1.5% of carbon dioxide and at least of the zinc vapor in the gaseous product is condensed as metallic zinc. For example, a typical gaseous product of an electrothermic zinc smelting furnace contains 40% Zn, 1.5% C02 and 51% CO', and has a reoxidation temperature of 941 C. The introduction of 3% of air (preheated to 600 C.) into the gaseous product, in accordance with the invention, raises the temperature of the product by about C. and yields a gas containing about 39% Zn, 2.7% CO2 and 49% CO. The reoxidation temperature of this resultant gaseous product is about 980 C., so that while the reoxidation temperature has been raised by 40 C., the actual temperature of the gaseous product is raised by 100 C., thus providing a safety factor of 60 C. against the fall in temperature of the gaseous product in its travel to the condenser.

The lower the initial amount of carbon dioxide in the gaseous product, the more is the reoxidation temperature raised by the introduction of a given amount of the oxygen-containing gas, and hence the less favorable is the effect of the introduction of the oxygen-containing gas in reducing the risk of oxidation of zinc vapor between the smelting furnace and the condenser. Accordingly, little advantage results from the practice of the invention where the carbon dioxide ntent of the gaseous product is much less than This application is a continuation-in-part of my application Ser. No. 166,339, filed June 6, 1950, now abandoned.

I claim:

1. In a process of smelting zinc ores in which the gaseous product contains at least 1.5% of carbon dioxide, the improvement for minimizing reoxidation of zinc vapor during the transfer of the gaseous product to a zinc vapor condenser which comprises directly introducing a controlled amount of an oxygen-containing gas into the gaseous product and thereby heating the gaseous product by the resulting oxidation of carbon monoxide to a temperature substantially above its normal initial temperature, and transferring the so-heated gaseous product to the condenser at a temperature above that at which 7 any appreciable amount of zinc vapor reacts with carbon dioxide.

2. In a process of smelting zinc ores in a blast furnace and removal of zinc vapor from the blast furnace gases by shock-chilling in a condenser, the improvement which comprises directly introducing a controlled amount of an oxygen-containing gas into the blast furnace gases and thereby heating the gases by the resulting oxidation of carbon monoxide to a temperature substantially above their normal initial temperature, and transferring the so-heated blast furnace gases to the shock-chilling zone of the condenser at a temperature above that at which any appreciable amount of zinc vapor reacts with carbon dioxide.

3. In the process of smelting zinc ores by means of a blast furnace and removing the zinc vapor from the blast furnace gases in a condenser, the improvement Which comprises directly introducing a controlled amount of an oxygen-containing gas into the blast furnace gases in a passage between and communicating with the blast furnace and the condenser to oxidize carbon monoxide contained in the blast furnace gases so that the heat of oxidation thus evolved serves to raise the temperature of the gases in order to reduce the formation of zinc oxide.

4. The process as claimed in claim 3 in which the oxygen-containing gas is introduced close lto the entrance from the furnace to said passage.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,885,412 Buskett Nov. 1, 1932 1,927,763 Wejnarth Sept. 19, 1933 1,983,025 Ginder et al. Dec. 4, 1934 2,086,354 Dorian July 6, 1937 2,492,438 Peirce Dec. 27, 1949 

1. IN A PROCESS OF SMELTING ZINC ORES IN WHICH THE GASEOUS PRODUCT CONTAINS AT LEAST 1.5% OF CARBON DIOXIDE, THE IMPROVEMENT FOR MINIMIZING REOXIDATION OF ZINC VAPOR DURING THE TRANSFER OF THE GASEOUS PRODUCT TO A ZINC VAPOR CONDENSER WHICH COMPRISES DIRECTLY INTRODUCING A CONTROLLED AMOUNT OF AN OXYGEN-CONTAINING GAS INTO THE GASES PRODUCT AND THEREBY HEATING THE GASEOUS PRODUCT BY THE RESULTING OXIDATION OF CARBON MONOXIDE TO A TEMPERATURE SUBSTANTIALLY ABOVE ITS NORMAL INITIAL TEMPERATURE, AND TRANSFERRING THE SO-HEATED GASEOUS PRODUCT TO THE CONDENSER AT A TEMPERATURE ABOVE THAT AT WHICH ANY APPRECIABLE AMOUNT OF ZINC VAPOR REACTS WITH CARBON DIOXIDE. 